Only because there's an agreement between propellant vendors and NAR/Tripoli who want to police what you do (commercial organizations) - no statute exists prohi

Only because there's an agreement between propellant vendors and NAR/Tripoli who want to police what you do (commercial organizations) - no statute exists prohibiting these activities (besides possibly ITAR and friends - which don't prohibit most domestic applications of this tech.) Motors and batteries are available from a very large number of suppliers and used for many other applications - no such single chokepoint exists to control drone users/manufacturers.